An ultrasonic fuel-gauging system for an aircraft is described in WO 2005/015134. An ultrasonic transducer is attached to a carrier tape which is coupled to an internal surface of the fuel tank. The transducer and tape are permanently fixed in place, and thus increase the weight of the aircraft. Also, the complexity of the system makes it difficult to retro-fit to an existing in-service aircraft.
An ultrasonic fuel level monitoring device is described in US 2005/0284218. In this case the transducer unit is externally mounted. During installation, a control unit on the top of the tank is configured to constantly send ping messages to the transducer unit to allow an installer to find the correct installation spot on the bottom of the tank. The transducer unit is then permanently affixed to the tank bottom at that exact location.